1. Field of the Invention
The present invention concerns a prepolymerized catalyst composition suitable for use in the polymerization of alpha-olefins and a process for preparing such a composition. The invention also relates to a process for polymerizing .alpha.-olefins with a Ziegler-Natta catalyst composition, according to which a procatalyst composition comprising a transition metal is prepolymerized with a monomer for producing a Ziegler-Natta type prepolymerized catalyst composition and this catalyst composition is contacted with an alpha-olefin monomer in order to produce a polymer.
2. Description of the Related Art
Ziegler-Natta catalysts have been used for a long time for stereospecific polymerization of alpha-olefins. In their broadest sense Ziegler-Natta catalysts comprise a compound, usually a halide, of a transition metal of groups IV to VIII of the periodic system together with a cocatalyst comprising an alkyl derivative of a group I to III metal. Titanium chlorides are representatives of typical transition metal compounds used, and of the metal alkyl compounds aluminium alkyl derivatives should be mentioned. Often a Ziegler-Natta catalyst also contains an electron donor, such as a Lewis base, the purpose of which is to render the polymerization product more isotactic. In order to improve the catalyst's activity, its surface is increased by adding the catalyst onto a support. The most common support is magnesium dichloride.
The activity of supported catalysts (calculated on basis of the weight of the transition metal) can be up to 100 times higher than the activity of unsupported catalysts. This is the reason why a growing interest is being focused on these catalysts. Supported Ziegler-Natta type catalysts can be prepared by depositing a transition metal compound, such as titanium tetrachloride, on a support, and then reducing the transition metal compound with an organic metal compound acting as a cocatalyst either before polymerization or at the beginning of polymerization. The deposited support is often dried before being subjected to reduction, but polymerization and reduction of the transition metal can be carried out directly after deposition of the catalyst on the support, which makes it possible to omit the drying step. This alternative is preferred if the supported catalyst is to be posttreated by prepolymerizing a suitable monomer on its surface, which is the case in, for instance, bulk polymerization of propene, wherein the catalyst is prepolymerized before being added into the polymerization reactor.
According to conventional technology, the Ziegler-Natta catalyst is used for polymerization in particulate form. In order to facilitate catalyst dosing the catalyst can be combined with wax which makes it possible to continuously to feed a prepolymerized wax catalyst into the polymerization reactor. By way of example, reference is made to GB Patent Application No. 2,189,252, which discloses a catalyst for the polymerization of 1-alkenes. Said prior art catalyst is prepared by depositing transition metal compounds on silica or alumina, a paraffinic hydrocarbon being applied to the catalyst together with a transition metal compound or to the completed catalyst. The paraffinic hydrocarbon can be prepared by prepolymerization of a small amount of 1-alkenes on the support.
The handling of wax catalysts is uncomplicated and safe during the various stages of the process. An additional benefit associated with wax protected catalysts resides in the fact that the catalyst is not as easily destroyed by external catalyst poisons as conventional catalysts. However, as a disadvantage, it should be noted that the process for preparing the wax catalysts comprises several stages; according to the prior art, the catalyst particles will first have to be dried before they can be mixed with the wax, in which they are prepolymerized. The GB Patent Application No. 2,189,252 mentioned above indicates that the solvent used during the addition of the wax has to be removed, e.g., by decanting, by evacuation or by evaporation.
The preparation reactions and the reactants of the Ziegler-Natta-catalysts are extremely sensitive to impurities. In particular, the drying of the catalyst is a sensitive stage during which the catalyst may easily be deactivated. For this reason there have been developed processes which comprise conducting the activation, washing, drying and prepolymerization stages in the same apparatus. In this context, reference is made to the equipment described in the U.S. Patent Specifications Nos. 5,137,551 and 5,215,949. Said references do not, however, deal with the preparation of prepolymerized catalysts which are fed in wax.